[Review] Request from 'vlewin' @ 'SUSE/connect/review_140409_deregister_system_with_connect'

Please review the following changes:
  * c59e69f add method for deregister call
  * 2e62385 test SUSE::Connect::Api.deregister response and status code
  * e8cbad8 add webmock for deregister call
  * beb0a9e add deregister/unregister a system to SUSE::Connect::Client
  * d3e6124 test SUSE::Connect::Client.deregister! method
  * 974eba0 add SUSE::Connect::System.remove_credentials
  * 267991c test remove_credentials SUSE::Connect::System method
  * 9a918a0 test empty response body
  * c655c7a don't fail if response body is empty